Effective catalytic treatment of internal combustion engine emissions requires precise control of engine air/fuel ratio. Even minor deviations in engine air/fuel ratio away from the stoichiometric ratio can lead to significant increase in at least one of the undesirable emissions components of hydrocarbons HC, carbon monoxide CO, and oxides of nitrogen NOx. Conventional air/fuel ratio control approaches estimate actual engine air/fuel ratio through at least one central air/fuel ratio sensor and vary central control commands in response thereto. The estimate of actual air/fuel ratio is assumed to represent the air/fuel ratio of all cylinders of multi-cylinder engines. A common control command is varied in response to the estimate of actual air/fuel ratio and is indiscriminately applied for control of all engine cylinders. Variations in the air/fuel ratio sensor output signal away from an average or single representative value are treated as "noise" and are filtered out of control processes to arrive at a single value representative of the sensor output over a number of engine cylinder combustion events.
There exist significant variations in the manner in which engine intake air and fuel are distributed between the cylinders of multi-cylinder engines and in the manner the air and fuel are consumed in each cylinder, for example, due to geometry variations between cylinders, intake runners, fuel injectors, etc., which result in flow variations between engine cylinders. Such distribution variations can result in significant variation in air/fuel ratio between engine cylinders. Much of the variation in the output signal of the air/fuel ratio sensor may therefore not be "noise" but may include information on fuel or air distribution variation between engine cylinders. Such distribution variation leads to transient departure of the engine air/fuel ratio away from a target air/fuel ratio, such as the stoichiometric ratio, which can lead to reduced catalytic treatment effectiveness and increased emissions.
It would therefore be desirable to translate air/fuel ratio measurement information into an indication of individual cylinder air/fuel ratio and to control air/fuel ratio on a cylinder-by-cylinder basis to further reduce engine emissions.